/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

package com.mlizhi.base.imageloader.core.assist.deque;

import java.util.AbstractQueue;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

/**
 * An optionally-bounded {@linkplain BlockingDeque blocking deque} based on
 * linked nodes.
 *
 * <p> The optional capacity bound constructor argument serves as a
 * way to prevent excessive expansion. The capacity, if unspecified,
 * is equal to {@link Integer#MAX_VALUE}.  Linked nodes are
 * dynamically created upon each insertion unless this would bring the
 * deque above capacity.
 *
 * <p>Most operations run in constant time (ignoring time spent
 * blocking).  Exceptions include {@link #remove(Object) remove},
 * {@link #removeFirstOccurrence removeFirstOccurrence}, {@link
 * #removeLastOccurrence removeLastOccurrence}, {@link #contains
 * contains}, {@link #iterator iterator.remove()}, and the bulk
 * operations, all of which run in linear time.
 *
 * <p>This class and its iterator implement all of the
 * <em>optional</em> methods of the {@link java.util.Collection} and {@link
 * java.util.Iterator} interfaces.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @since 1.6
 * @author  Doug Lea
 * @param <E> the type of elements held in this collection
 */
public class LinkedBlockingDeque<E>
extends AbstractQueue<E>
implements BlockingDeque<E>,  java.io.Serializable {

/*
 * Implemented as a simple doubly-linked list protected by a
 * single lock and using conditions to manage blocking.
 *
 * To implement weakly consistent iterators, it appears we need to
 * keep all Nodes GC-reachable from a predecessor dequeued Node.
 * That would cause two problems:
 * - allow a rogue Iterator to cause unbounded memory retention
 * - cause cross-generational linking of old Nodes to new Nodes if
 *   a Node was tenured while live, which generational GCs have a
 *   hard time dealing with, causing repeated major collections.
 * However, only non-deleted Nodes need to be reachable from
 * dequeued Nodes, and reachability does not necessarily have to
 * be of the kind understood by the GC.  We use the trick of
 * linking a Node that has just been dequeued to itself.  Such a
 * self-link implicitly means to jump to "first" (for next links)
 * or "last" (for prev links).
 */

/*
 * We have "diamond" multiple interface/abstract class inheritance
 * here, and that introduces ambiguities. Often we want the
 * BlockingDeque javadoc combined with the AbstractQueue
 * implementation, so a lot of method specs are duplicated here.
 */

private static final long serialVersionUID = -387911632671998426L;

/** Doubly-linked list node class */
static final class Node<E> {
/**
 * The item, or null if this node has been removed.
 */
E item;

/**
 * One of:
 * - the real predecessor Node
 * - this Node, meaning the predecessor is tail
 * - null, meaning there is no predecessor
 */
Node<E> prev;

/**
 * One of:
 * - the real successor Node
 * - this Node, meaning the successor is head
 * - null, meaning there is no successor
 */
Node<E> next;

Node(E x) {
item = x;
}
}

/**
 * Pointer to first node.
 * Invariant: (first == null && last == null) ||
 *(first.prev == null && first.item != null)
 */
transient Node<E> first;

/**
 * Pointer to last node.
 * Invariant: (first == null && last == null) ||
 *(last.next == null && last.item != null)
 */
transient Node<E> last;

/** Number of items in the deque */
private transient int count;

/** Maximum number of items in the deque */
private final int capacity;

/** Main lock guarding all access */
final ReentrantLock lock = new ReentrantLock();

/** Condition for waiting takes */
private final Condition notEmpty = lock.newCondition();

/** Condition for waiting puts */
private final Condition notFull = lock.newCondition();

/**
 * Creates a {@code LinkedBlockingDeque} with a capacity of
 * {@link Integer#MAX_VALUE}.
 */
public LinkedBlockingDeque() {
this(Integer.MAX_VALUE);
}

/**
 * Creates a {@code LinkedBlockingDeque} with the given (fixed) capacity.
 *
 * @param capacity the capacity of this deque
 * @throws IllegalArgumentException if {@code capacity} is less than 1
 */
public LinkedBlockingDeque(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
}

/**
 * Creates a {@code LinkedBlockingDeque} with a capacity of
 * {@link Integer#MAX_VALUE}, initially containing the elements of
 * the given collection, added in traversal order of the
 * collection's iterator.
 *
 * @param c the collection of elements to initially contain
 * @throws NullPointerException if the specified collection or any
 * of its elements are null
 */
public LinkedBlockingDeque(Collection<? extends E> c) {
this(Integer.MAX_VALUE);
final ReentrantLock lock = this.lock;
lock.lock(); // Never contended, but necessary for visibility
try {
for (E e : c) {
if (e == null)
throw new NullPointerException();
if (!linkLast(new Node<E>(e)))
throw new IllegalStateException("Deque full");
}
} finally {
lock.unlock();
}
}


// Basic linking and unlinking operations, called only while holding lock

/**
 * Links node as first element, or returns false if full.
 */
private boolean linkFirst(Node<E> node) {
// assert lock.isHeldByCurrentThread();
if (count >= capacity)
return false;
Node<E> f = first;
node.next = f;
first = node;
if (last == null)
last = node;
else
f.prev = node;
++count;
notEmpty.signal();
return true;
}

/**
 * Links node as last element, or returns false if full.
 */
private boolean linkLast(Node<E> node) {
// assert lock.isHeldByCurrentThread();
if (count >= capacity)
return false;
Node<E> l = last;
node.prev = l;
last = node;
if (first == null)
first = node;
else
l.next = node;
++count;
notEmpty.signal();
return true;
}

/**
 * Removes and returns first element, or null if empty.
 */
private E unlinkFirst() {
// assert lock.isHeldByCurrentThread();
Node<E> f = first;
if (f == null)
return null;
Node<E> n = f.next;
E item = f.item;
f.item = null;
f.next = f; // help GC
first = n;
if (n == null)
last = null;
else
n.prev = null;
--count;
notFull.signal();
return item;
}

/**
 * Removes and returns last element, or null if empty.
 */
private E unlinkLast() {
// assert lock.isHeldByCurrentThread();
Node<E> l = last;
if (l == null)
return null;
Node<E> p = l.prev;
E item = l.item;
l.item = null;
l.prev = l; // help GC
last = p;
if (p == null)
first = null;
else
p.next = null;
--count;
notFull.signal();
return item;
}

/**
 * Unlinks x.
 */
void unlink(Node<E> x) {
// assert lock.isHeldByCurrentThread();
Node<E> p = x.prev;
Node<E> n = x.next;
if (p == null) {
unlinkFirst();
} else if (n == null) {
unlinkLast();
} else {
p.next = n;
n.prev = p;
x.item = null;
// Don't mess with x's links.  They may still be in use by
// an iterator.
--count;
notFull.signal();
}
}

// BlockingDeque methods

/**
 * @throws IllegalStateException {@inheritDoc}
 * @throws NullPointerException  {@inheritDoc}
 */
public void addFirst(E e) {
if (!offerFirst(e))
throw new IllegalStateException("Deque full");
}

/**
 * @throws IllegalStateException {@inheritDoc}
 * @throws NullPointerException  {@inheritDoc}
 */
public void addLast(E e) {
if (!offerLast(e))
throw new IllegalStateException("Deque full");
}

/**
 * @throws NullPointerException {@inheritDoc}
 */
public boolean offerFirst(E e) {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
return linkFirst(node);
} finally {
lock.unlock();
}
}

/**
 * @throws NullPointerException {@inheritDoc}
 */
public boolean offerLast(E e) {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
return linkLast(node);
} finally {
lock.unlock();
}
}

/**
 * @throws NullPointerException {@inheritDoc}
 * @throws InterruptedException {@inheritDoc}
 */
public void putFirst(E e) throws InterruptedException {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
while (!linkFirst(node))
notFull.await();
} finally {
lock.unlock();
}
}

/**
 * @throws NullPointerException {@inheritDoc}
 * @throws InterruptedException {@inheritDoc}
 */
public void putLast(E e) throws InterruptedException {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
while (!linkLast(node))
notFull.await();
} finally {
lock.unlock();
}
}

/**
 * @throws NullPointerException {@inheritDoc}
 * @throws InterruptedException {@inheritDoc}
 */
public boolean offerFirst(E e, long timeout, TimeUnit unit)
throws InterruptedException {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (!linkFirst(node)) {
if (nanos <= 0)
return false;
nanos = notFull.awaitNanos(nanos);
}
return true;
} finally {
lock.unlock();
}
}

/**
 * @throws NullPointerException {@inheritDoc}
 * @throws InterruptedException {@inheritDoc}
 */
public boolean offerLast(E e, long timeout, TimeUnit unit)
throws InterruptedException {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (!linkLast(node)) {
if (nanos <= 0)
return false;
nanos = notFull.awaitNanos(nanos);
}
return true;
} finally {
lock.unlock();
}
}

/**
 * @throws java.util.NoSuchElementException {@inheritDoc}
 */
public E removeFirst() {
E x = pollFirst();
if (x == null) throw new NoSuchElementException();
return x;
}

/**
 * @throws java.util.NoSuchElementException {@inheritDoc}
 */
public E removeLast() {
E x = pollLast();
if (x == null) throw new NoSuchElementException();
return x;
}

public E pollFirst() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return unlinkFirst();
} finally {
lock.unlock();
}
}

public E pollLast() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return unlinkLast();
} finally {
lock.unlock();
}
}

public E takeFirst() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lock();
try {
E x;
while ( (x = unlinkFirst()) == null)
notEmpty.await();
return x;
} finally {
lock.unlock();
}
}

public E takeLast() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lock();
try {
E x;
while ( (x = unlinkLast()) == null)
notEmpty.await();
return x;
} finally {
lock.unlock();
}
}

public E pollFirst(long timeout, TimeUnit unit)
throws InterruptedException {
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
E x;
while ( (x = unlinkFirst()) == null) {
if (nanos <= 0)
return null;
nanos = notEmpty.awaitNanos(nanos);
}
return x;
} finally {
lock.unlock();
}
}

public E pollLast(long timeout, TimeUnit unit)
throws InterruptedException {
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
E x;
while ( (x = unlinkLast()) == null) {
if (nanos <= 0)
return null;
nanos = notEmpty.awaitNanos(nanos);
}
return x;
} finally {
lock.unlock();
}
}

/**
 * @throws java.util.NoSuchElementException {@inheritDoc}
 */
public E getFirst() {
E x = peekFirst();
if (x == null) throw new NoSuchElementException();
return x;
}

/**
 * @throws java.util.NoSuchElementException {@inheritDoc}
 */
public E getLast() {
E x = peekLast();
if (x == null) throw new NoSuchElementException();
return x;
}

public E peekFirst() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return (first == null) ? null : first.item;
} finally {
lock.unlock();
}
}

public E peekLast() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return (last == null) ? null : last.item;
} finally {
lock.unlock();
}
}

public boolean removeFirstOccurrence(Object o) {
if (o == null) return false;
final ReentrantLock lock = this.lock;
lock.lock();
try {
for (Node<E> p = first; p != null; p = p.next) {
if (o.equals(p.item)) {
unlink(p);
return true;
}
}
return false;
} finally {
lock.unlock();
}
}

public boolean removeLastOccurrence(Object o) {
if (o == null) return false;
final ReentrantLock lock = this.lock;
lock.lock();
try {
for (Node<E> p = last; p != null; p = p.prev) {
if (o.equals(p.item)) {
unlink(p);
return true;
}
}
return false;
} finally {
lock.unlock();
}
}

// BlockingQueue methods

/**
 * Inserts the specified element at the end of this deque unless it would
 * violate capacity restrictions.  When using a capacity-restricted deque,
 * it is generally preferable to use method {@link #offer offer}.
 *
 * <p>This method is equivalent to {@link #addLast}.
 *
 * @throws IllegalStateException if the element cannot be added at this
 * time due to capacity restrictions
 * @throws NullPointerException if the specified element is null
 */
public boolean add(E e) {
addLast(e);
return true;
}

/**
 * @throws NullPointerException if the specified element is null
 */
public boolean offer(E e) {
return offerLast(e);
}

/**
 * @throws NullPointerException {@inheritDoc}
 * @throws InterruptedException {@inheritDoc}
 */
public void put(E e) throws InterruptedException {
putLast(e);
}

/**
 * @throws NullPointerException {@inheritDoc}
 * @throws InterruptedException {@inheritDoc}
 */
public boolean offer(E e, long timeout, TimeUnit unit)
throws InterruptedException {
return offerLast(e, timeout, unit);
}

/**
 * Retrieves and removes the head of the queue represented by this deque.
 * This method differs from {@link #poll poll} only in that it throws an
 * exception if this deque is empty.
 *
 * <p>This method is equivalent to {@link #removeFirst() removeFirst}.
 *
 * @return the head of the queue represented by this deque
 * @throws java.util.NoSuchElementException if this deque is empty
 */
public E remove() {
return removeFirst();
}

public E poll() {
return pollFirst();
}

public E take() throws InterruptedException {
return takeFirst();
}

public E poll(long timeout, TimeUnit unit) throws InterruptedException {
return pollFirst(timeout, unit);
}

/**
 * Retrieves, but does not remove, the head of the queue represented by
 * this deque.  This method differs from {@link #peek peek} only in that
 * it throws an exception if this deque is empty.
 *
 * <p>This method is equivalent to {@link #getFirst() getFirst}.
 *
 * @return the head of the queue represented by this deque
 * @throws java.util.NoSuchElementException if this deque is empty
 */
public E element() {
return getFirst();
}

public E peek() {
return peekFirst();
}

/**
 * Returns the number of additional elements that this deque can ideally
 * (in the absence of memory or resource constraints) accept without
 * blocking. This is always equal to the initial capacity of this deque
 * less the current {@code size} of this deque.
 *
 * <p>Note that you <em>cannot</em> always tell if an attempt to insert
 * an element will succeed by inspecting {@code remainingCapacity}
 * because it may be the case that another thread is about to
 * insert or remove an element.
 */
public int remainingCapacity() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return capacity - count;
} finally {
lock.unlock();
}
}

/**
 * @throws UnsupportedOperationException {@inheritDoc}
 * @throws ClassCastException{@inheritDoc}
 * @throws NullPointerException  {@inheritDoc}
 * @throws IllegalArgumentException  {@inheritDoc}
 */
public int drainTo(Collection<? super E> c) {
return drainTo(c, Integer.MAX_VALUE);
}

/**
 * @throws UnsupportedOperationException {@inheritDoc}
 * @throws ClassCastException{@inheritDoc}
 * @throws NullPointerException  {@inheritDoc}
 * @throws IllegalArgumentException  {@inheritDoc}
 */
public int drainTo(Collection<? super E> c, int maxElements) {
if (c == null)
throw new NullPointerException();
if (c == this)
throw new IllegalArgumentException();
final ReentrantLock lock = this.lock;
lock.lock();
try {
int n = Math.min(maxElements, count);
for (int i = 0; i < n; i++) {
c.add(first.item);   // In this order, in case add() throws.
unlinkFirst();
}
return n;
} finally {
lock.unlock();
}
}

// Stack methods

/**
 * @throws IllegalStateException {@inheritDoc}
 * @throws NullPointerException  {@inheritDoc}
 */
public void push(E e) {
addFirst(e);
}

/**
 * @throws java.util.NoSuchElementException {@inheritDoc}
 */
public E pop() {
return removeFirst();
}

// Collection methods

/**
 * Removes the first occurrence of the specified element from this deque.
 * If the deque does not contain the element, it is unchanged.
 * More formally, removes the first element {@code e} such that
 * {@code o.equals(e)} (if such an element exists).
 * Returns {@code true} if this deque contained the specified element
 * (or equivalently, if this deque changed as a result of the call).
 *
 * <p>This method is equivalent to
 * {@link #removeFirstOccurrence(Object) removeFirstOccurrence}.
 *
 * @param o element to be removed from this deque, if present
 * @return {@code true} if this deque changed as a result of the call
 */
public boolean remove(Object o) {
return removeFirstOccurrence(o);
}

/**
 * Returns the number of elements in this deque.
 *
 * @return the number of elements in this deque
 */
public int size() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return count;
} finally {
lock.unlock();
}
}

/**
 * Returns {@code true} if this deque contains the specified element.
 * More formally, returns {@code true} if and only if this deque contains
 * at least one element {@code e} such that {@code o.equals(e)}.
 *
 * @param o object to be checked for containment in this deque
 * @return {@code true} if this deque contains the specified element
 */
public boolean contains(Object o) {
if (o == null) return false;
final ReentrantLock lock = this.lock;
lock.lock();
try {
for (Node<E> p = first; p != null; p = p.next)
if (o.equals(p.item))
return true;
return false;
} finally {
lock.unlock();
}
}

/*
 * TODO: Add support for more efficient bulk operations.
 *
 * We don't want to acquire the lock for every iteration, but we
 * also want other threads a chance to interact with the
 * collection, especially when count is close to capacity.
 */

// /**
//  * Adds all of the elements in the specified collection to this
//  * queue.  Attempts to addAll of a queue to itself result in
//  * {@code IllegalArgumentException}. Further, the behavior of
//  * this operation is undefined if the specified collection is
//  * modified while the operation is in progress.
//  *
//  * @param c collection containing elements to be added to this queue
//  * @return {@code true} if this queue changed as a result of the call
//  * @throws ClassCastException{@inheritDoc}
//  * @throws NullPointerException  {@inheritDoc}
//  * @throws IllegalArgumentException  {@inheritDoc}
//  * @throws IllegalStateException {@inheritDoc}
//  * @see #add(Object)
//  */
// public boolean addAll(Collection<? extends E> c) {
// if (c == null)
// throw new NullPointerException();
// if (c == this)
// throw new IllegalArgumentException();
// final ReentrantLock lock = this.lock;
// lock.lock();
// try {
// boolean modified = false;
// for (E e : c)
// if (linkLast(e))
// modified = true;
// return modified;
// } finally {
// lock.unlock();
// }
// }

/**
 * Returns an array containing all of the elements in this deque, in
 * proper sequence (from first to last element).
 *
 * <p>The returned array will be "safe" in that no references to it are
 * maintained by this deque.  (In other words, this method must allocate
 * a new array).  The caller is thus free to modify the returned array.
 *
 * <p>This method acts as bridge between array-based and collection-based
 * APIs.
 *
 * @return an array containing all of the elements in this deque
 */
public Object[] toArray() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
Object[] a = new Object[count];
int k = 0;
for (Node<E> p = first; p != null; p = p.next)
a[k++] = p.item;
return a;
} finally {
lock.unlock();
}
}

/**
 * Returns an array containing all of the elements in this deque, in
 * proper sequence; the runtime type of the returned array is that of
 * the specified array.  If the deque fits in the specified array, it
 * is returned therein.  Otherwise, a new array is allocated with the
 * runtime type of the specified array and the size of this deque.
 *
 * <p>If this deque fits in the specified array with room to spare
 * (i.e., the array has more elements than this deque), the element in
 * the array immediately following the end of the deque is set to
 * {@code null}.
 *
 * <p>Like the {@link #toArray()} method, this method acts as bridge between
 * array-based and collection-based APIs.  Further, this method allows
 * precise control over the runtime type of the output array, and may,
 * under certain circumstances, be used to save allocation costs.
 *
 * <p>Suppose {@code x} is a deque known to contain only strings.
 * The following code can be used to dump the deque into a newly
 * allocated array of {@code String}:
 *
 * <pre>
 * String[] y = x.toArray(new String[0]);</pre>
 *
 * Note that {@code toArray(new Object[0])} is identical in function to
 * {@code toArray()}.
 *
 * @param a the array into which the elements of the deque are to
 *  be stored, if it is big enough; otherwise, a new array of the
 *  same runtime type is allocated for this purpose
 * @return an array containing all of the elements in this deque
 * @throws ArrayStoreException if the runtime type of the specified array
 * is not a supertype of the runtime type of every element in
 * this deque
 * @throws NullPointerException if the specified array is null
 */
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
final ReentrantLock lock = this.lock;
lock.lock();
try {
if (a.length < count)
a = (T[])java.lang.reflect.Array.newInstance
(a.getClass().getComponentType(), count);

int k = 0;
for (Node<E> p = first; p != null; p = p.next)
a[k++] = (T)p.item;
if (a.length > k)
a[k] = null;
return a;
} finally {
lock.unlock();
}
}

public String toString() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
Node<E> p = first;
if (p == null)
return "[]";

StringBuilder sb = new StringBuilder();
sb.append('[');
for (;;) {
E e = p.item;
sb.append(e == this ? "(this Collection)" : e);
p = p.next;
if (p == null)
return sb.append(']').toString();
sb.append(',').append(' ');
}
} finally {
lock.unlock();
}
}

/**
 * Atomically removes all of the elements from this deque.
 * The deque will be empty after this call returns.
 */
public void clear() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
for (Node<E> f = first; f != null; ) {
f.item = null;
Node<E> n = f.next;
f.prev = null;
f.next = null;
f = n;
}
first = last = null;
count = 0;
notFull.signalAll();
} finally {
lock.unlock();
}
}

/**
 * Returns an iterator over the elements in this deque in proper sequence.
 * The elements will be returned in order from first (head) to last (tail).
 *
 * <p>The returned iterator is a "weakly consistent" iterator that
 * will never throw {@link java.util.ConcurrentModificationException
 * ConcurrentModificationException}, and guarantees to traverse
 * elements as they existed upon construction of the iterator, and
 * may (but is not guaranteed to) reflect any modifications
 * subsequent to construction.
 *
 * @return an iterator over the elements in this deque in proper sequence
 */
public Iterator<E> iterator() {
return new Itr();
}

/**
 * Returns an iterator over the elements in this deque in reverse
 * sequential order.  The elements will be returned in order from
 * last (tail) to first (head).
 *
 * <p>The returned iterator is a "weakly consistent" iterator that
 * will never throw {@link java.util.ConcurrentModificationException
 * ConcurrentModificationException}, and guarantees to traverse
 * elements as they existed upon construction of the iterator, and
 * may (but is not guaranteed to) reflect any modifications
 * subsequent to construction.
 *
 * @return an iterator over the elements in this deque in reverse order
 */
public Iterator<E> descendingIterator() {
return new DescendingItr();
}

/**
 * Base class for Iterators for LinkedBlockingDeque
 */
private abstract class AbstractItr implements Iterator<E> {
/**
 * The next node to return in next()
 */
 Node<E> next;

/**
 * nextItem holds on to item fields because once we claim that
 * an element exists in hasNext(), we must return item read
 * under lock (in advance()) even if it was in the process of
 * being removed when hasNext() was called.
 */
E nextItem;

/**
 * Node returned by most recent call to next. Needed by remove.
 * Reset to null if this element is deleted by a call to remove.
 */
private Node<E> lastRet;

abstract Node<E> firstNode();
abstract Node<E> nextNode(Node<E> n);

AbstractItr() {
// set to initial position
final ReentrantLock lock = LinkedBlockingDeque.this.lock;
lock.lock();
try {
next = firstNode();
nextItem = (next == null) ? null : next.item;
} finally {
lock.unlock();
}
}

/**
 * Returns the successor node of the given non-null, but
 * possibly previously deleted, node.
 */
private Node<E> succ(Node<E> n) {
// Chains of deleted nodes ending in null or self-links
// are possible if multiple interior nodes are removed.
for (;;) {
Node<E> s = nextNode(n);
if (s == null)
return null;
else if (s.item != null)
return s;
else if (s == n)
return firstNode();
else
n = s;
}
}

/**
 * Advances next.
 */
void advance() {
final ReentrantLock lock = LinkedBlockingDeque.this.lock;
lock.lock();
try {
// assert next != null;
next = succ(next);
nextItem = (next == null) ? null : next.item;
} finally {
lock.unlock();
}
}

public boolean hasNext() {
return next != null;
}

public E next() {
if (next == null)
throw new NoSuchElementException();
lastRet = next;
E x = nextItem;
advance();
return x;
}

public void remove() {
Node<E> n = lastRet;
if (n == null)
throw new IllegalStateException();
lastRet = null;
final ReentrantLock lock = LinkedBlockingDeque.this.lock;
lock.lock();
try {
if (n.item != null)
unlink(n);
} finally {
lock.unlock();
}
}
}

/** Forward iterator */
private class Itr extends AbstractItr {
Node<E> firstNode() { return first; }
Node<E> nextNode(Node<E> n) { return n.next; }
}

/** Descending iterator */
private class DescendingItr extends AbstractItr {
Node<E> firstNode() { return last; }
Node<E> nextNode(Node<E> n) { return n.prev; }
}

/**
 * Save the state of this deque to a stream (that is, serialize it).
 *
 * @serialData The capacity (int), followed by elements (each an
 * {@code Object}) in the proper order, followed by a null
 * @param s the stream
 */
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
final ReentrantLock lock = this.lock;
lock.lock();
try {
// Write out capacity and any hidden stuff
s.defaultWriteObject();
// Write out all elements in the proper order.
for (Node<E> p = first; p != null; p = p.next)
s.writeObject(p.item);
// Use trailing null as sentinel
s.writeObject(null);
} finally {
lock.unlock();
}
}

/**
 * Reconstitute this deque from a stream (that is,
 * deserialize it).
 * @param s the stream
 */
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
count = 0;
first = null;
last = null;
// Read in all elements and place in queue
for (;;) {
@SuppressWarnings("unchecked")
E item = (E)s.readObject();
if (item == null)
break;
add(item);
}
}

}
